Lawyers for Samantha Orobator, the heavily pregnant British woman who narrowly escaped the death penalty in Laos, today condemned the Ministry of Justice for insisting she serve a life sentence in Britain despite being convicted under coercion and without access to lawyers.

Orobator, 20, landed back in London this morning after being transferred from a Laotian jail to British custody under a treaty that agreed to enforce her life sentence for smuggling heroin. She was taken straight to Holloway prison in north London.

In a statement, Orobator, whose child is due next month, said she was "enormously relieved and happy to be back on British soil", describing her ordeal as "an unimaginable nightmare" and "a traumatic experience for me and for my unborn child".

Her mother, Jane, said she was "really happy and delighted that [Samantha] is back where she belongs", adding that she felt like screaming with joy. "I am very relieved that at last she will be able to access good medical attention, not for her sake alone but for the sake of her unborn baby." The Londoner received no antenatal care through the eight months of her pregnancy.

The circumstances by which Orobator conceived her child, while in the female-only wing of Phonthong prison in the Lao capital, Vientiane, remain unclear; early reports that she had been raped by a guard were followed by suggestions that she had deliberately tried to escape the death penalty by smuggling semen from another British prisoner, John Watson, who has been serving a life sentence since 2003 for smuggling amphetamines.

Clive Stafford Smith, director of the legal human rights charity Reprieve, said Orobator had not had an opportunity to discuss this "private" matter with her family or lawyers, "but you can take it for granted that nothing that happens to a young woman in those circumstances could truly be said to be voluntary in any way". He disputed the suggestion that the pregnancy was a deliberate attempt to avoid a firing squad, saying the trial had originally been scheduled for August 2010, when her baby would have been born and she was once again eligible for the death penalty. He described her as "a first-time young mother-to-be, sitting in some godawful prison being threatened with being taken out and shot".

In her year in prison, Orobator was never permitted to meet lawyers in private and had been told she would not receive a trial unless she signed "ridiculous" documents about the paternity of her baby, according to Reprieve. At her trial she was not permitted to enter a plea, was questioned only by the prosecution and was unable to call witnesses, being sentenced in just half an hour.

"The idea that she should be held for the rest of her life on this basis would be laughable if it wasn't true," said Stafford Smith. "We call on Jack Straw and the Ministry of Justice to make it very clear immediately that this will not be [the case]. If that's not done you can rest assured that there will be legal proceedings against the British government."

Orobator was arrested on 6 August last year at Wattay airport in Vientiane and convicted in June of smuggling 680g of heroin. Asked about the circumstances of her arrest, Stafford Smith said: "For 800 years in this country we have presumed people innocent until they have had a fair trial, and we can certainly say in this case that it wasn't fair … [but] Reprieve's independent investigations suggest that she's not guilty of any criminal offence."

Stafford Smith said that her trial in Laos had been a "total sham". A brief meeting with two Reprieve lawyers today had been the first time Orobator had seen an independent lawyer in private, he said, adding that she had been repeatedly threatened with death by firing squad even while pregnant, forced to sign contradictory statements and denied access to consular assistance. "To call the Lao legal proceedings a kangaroo court would be an offence to the kangaroo family," he said.

"The UK will continue to enforce the sentence imposed on her by the Lao courts," a ministry spokeswoman said. "Samantha Orabator was tried in accordance with Lao law and we're not making any further comment." It is understood that it will seek a high court hearing to determine the life sentence tariff that Orobator, from Peckham in south London, will be obliged to serve.

Inside the jail

Samantha Orobator is "as well as can be expected" after a year in Phonthong prison, her lawyers said today. Details remain sketchy of the conditions in which she was held. Former inmates of the jail, however, have described a prison regime in which six prisoners share "dingy, stinking" cells measuring four metres by four metres, and daily rations reportedly consist of two bowls of pig-fat, watery soup and a bowl of sticky rice.

Kay Danes, an Australian woman who spent 11 months in Phonthong with her husband in 2002 for what she insists were trumped up charges of stealing gemstones, wrote a book about the jail in which she said beatings, interrogation and even torture were part of daily life, even though Phonthong is considered a "VIP" prison compared with the much more crowded domestic jails.

Her cell, more comfortable than most, had a hole in the ground for a toilet and a few blankets on wooden floorboards as bedding. Some prisoners spent a year in their cells without being allowed out for exercise, she said.